9.2.5.5 Heap Cell Type Information

Heap cells contain a number of entries, each of which is either a scheme
object of type SCM or a raw C value of type scm_t_bits.
Which of the cell entries contain Scheme objects and which contain raw C
values is determined by the first entry of the cell, which holds the
cell type information.

Macro: scm_t_bitsSCM_CELL_TYPE(SCM x)

For a non-immediate Scheme object x, deliver the content of the
first entry of the heap cell referenced by x. This value holds
the information about the cell type.

Macro: voidSCM_SET_CELL_TYPE(SCM x, scm_t_bits t)

For a non-immediate Scheme object x, write the value t into
the first entry of the heap cell referenced by x. The value
t must hold a valid cell type.